Acoustic-wave apparatus



Oct. 17, 1967 a G WATTERS ET AL, 3,347,335

ACOUSTIC-WAVE APPARATUS I Original Filed March 24, 1959 4 Sheets-Shet 1Fig 1 PRIOR/1R! 000 P191012 Aer BEND/N6 WAVE VflOC/TV 11%;

INVENTORfi BILL e. WATTERS 1g. 3 GUENTHER KURTZE M BY Ema/2d ATTORNEYS()cf. 17, 1967 WATTERS ET AL 3,347,335

ACOUSTlC WAVE APPARATUS INVENTOR BILL G. WATTERS GUENTHER KURTZEATTORNEYS Oct. 17, 1967 B G WATTERS ET AL 3,347,335.

. ACOUSTIC-WAVE APPARATUS Original Filed March 24, 1959 4 Sheets-Sheei 3.1 I I f' INVENTQRS am. e. WATTERS GUENTHER KURTZE ATTORNEYS Oct. 17,1967 a WATERS ET AL 3,347,335

ACOUSTlC-WAYE APPARATUS Original Filed March 24,.1959 4 Sheets-Sheet 4 Fi] H H H H H H H II H H H H H H H H H Hli H H H'H H H H 1 H H H II H H HH H H H H H H II H H H HH "WW I INVENTORS anu. G- WATTERS BY GUENTHERKURTZE ATTORNEYS United States Patent C) Claims. (Cl. 181-.5)

This application is a continuation of Ser. No. 801,653, filed Mar. 24,1959, now abandoned, for Acoustic Wave Apparatus.

The present invention relates to acoustic-Wave vibrational radiating orreceiving apparatus and, more particularly, though not exclusively, toelectro-acoustic transducers. The terms acoustic, sound and vibration,as employed herein, are used in their generic sense to connote not onlyaudible, but super and sub-audibile elastic vibrations, as well.

It has previously been recognized that vibrational bending wavespropagating along a plate or bar will radiate acoustic waves into thesurrounding medium, such as air or water, provided the velocity of thebending waves in the plate or bar exceeds the velocity of sound in themedium. In the case of a plane bending wave propagating in a plate ofinfinite size, a plane sound Wave is radiated into the medium at anangle of elevation given by the expression,

cos =c /c (1) where c is the velocity in the surrounding medium, and athe velocity of bending waves in the plate.

The use of this phenomenon for the development of directive transducersis described, for example, in United States Letters Patent No.2,063,945, issued Dec. 15, 1936, to George W. Pierce, wherein a drivingsystem is coupled to one of the ends of a plate or bar. When a plate orbar of finite length is so used, a non-reflecting termination at the endopposite to the driving system is necessary since Waves reflected fromthe said opposite end will generate radiation under the angle ofelevation of 180 as well. It is also to be understood that, though thedescription may proceed upon the basis of the illustrated examples ofdriving systems coupled to the plate or bar and the like, the same typeof performance is achieved in receiving systems where the driving systemis replaced by a vibration pick-up system.

Assuming the Width of the plate or bar to be small compared with thewavelength of the vibrations, and assuming, further, that only one sideof the strip is exposed i to the surrounding medium (as when the otherside is covered by a housing), the acoustic radiation or receptionpattern will assume the shape of a cone around the axis of the plate orbar with a total vertex angle of 2gb. If, however, both of the radiatingsides of the plate or bar are exposed to the acoustic field, afigure-eight-shaped dipolecharacteristic is superimposed in planesperpendicular to the plate or bar axis, and no radiation or reception isobserved in the plane of the plate or bar itself. 3 Thewidth of theradiation maximum in a plane containing the axis of the plate or bar,usually expressed by the spatial angle within which the radiationamplitude is within three decibels from maximum, is a function of theratio of plate or bar length L and the bending wavelength A andgenerally decreases as this ratio increases. A length L= is about theminimum length necessary to obtain pronounced directivity. For shorterplates or bars, or for larger Wavelengths, the radiation or receptionpattern of a piston source is approached and the radiation no longerdepends upon the beforeementioned condition that c be larger than c Theequation where f is the frequency of the bending vibrations, hencedetermines the lower frequency limit of a directive device of this kind.The upper frequency limit is usually given by the width w of the plateor bar, since the radia tion or reception pattern is radically changedwhen the wavelength becomes smaller than 2w; i.e..

The main handicap towards practical use of a directive transducer ofthis type, however, resides in the fact that the velocity of the bendingwaves increases with the square root of frequency FV V (4) where B isthe bending stiffness of the plate or bar and M is the mass per unitarea thereof. Hence the angle of maximum radiation o is a function offrequency, so that the di-rectivity pattern changes with frequency.

It is accordingly an object of the present invention to adapt suchplates or bars or the like for broad-band operation throughconstructional features that provide for an angle of maximum radiationthat remains independent of frequency over a wide frequency'band.

A further object is to provide a new and improved radiating or receivingapparatus.

An additional object is to provide -a novel transducer.

Other and further objects will be explained hereinafter and will be moreparticularly pointed out inconnection with the appended claims.

The invention will now be described in connection with the accompanyingdrawing, FIGURE 1 of which is a side elevation of a plate or barvibrating in the manner beforedescribed to provide for directionalacoustic radiation or reception;

FIGURE 2 is a perspective view, partly broken away to illustrate detailsof construction, of a preferred embodiment of the broad-band transducerof the present invention;

FIGURES 3 and 4 are graphs contrasting the performance of the presentinvention with that of the prior art;

FIGURES 5, 6, 7 and 9 are views similar to FIGURE 1 of modifications;and

FIGURE 8 is a top elevation illustrating the invention employed forstereophonic purposes.

Referring to FIGURE 1, a plate, bar or the like 1, as described in thesaid Letters Patent, is shown deformed by bending waves of wavelength Ain the plate or bar 1. The wavelength A is greater than the wavelength Aof acoustic waves in the surrounding medium, such as air or water, sothat acoustic radiation or reception occurs directionally at the angle5, as given by Equation 1, above, and as indicated by the dash-linedwave fronts. The bending waves may be generated by an electromechanicaldriver 3, such as a magnetomotive or magnetostrictive vibrator, showndisposed near an end of the plate or bar 1. Other types of vibratorydevices, including piezoelectric apparatus, may be coupled to the plateor bar 1 to generate the vibrations therein. Conversely, the plate orbar 1 may be set into vibration by an incident acoustic wave and themember 3 may transduce the mechanical vibrations into electrical signalsfor reception purposes.

As above explained, as the frequency of the vibrations is changed, thevelocity of the bending waves and the angle of radiation or receptionalso change. The squareroot variation of bending-wave velocity withfrequency (Equation 4) is plotted in the dash-line graph of FIGURE 3,labelled Prior art, frequency being indicated along the abscissa inunits of cycles per second, and bendingwave velocity being plotted alongthe ordinate in units of meters per second. When the velocity exceedsthe hori- Zontal dash line labelled Velocity of Sound in Air, thedirective radiation or reception phenomenon occurs. The variation of theangle 1: of radiation or reception is similarly plotted in the dash-linecurve of FIGURE 4. Clearly, then, if the apparatus of FIGURE 1 were tobe employed with multi-frequency waves, the radiation or receptionpattern would shift for the different frequencies. This renders thedevice unsuitable for such purposes, for example, as loudspeakeroperation and the like.

In accordance with the present invention, however, it has been foundthat appropriately constructed multi-layer plates, strips or the likecan be caused to follow the square-root law of Equation 4 only up to acertain critical frequency i FIGURE 3; whereas above this frequency f,,,the velocity of bending waves remains substantially constant at I over abroad band of frequencies. By proper choice of parameters, theperformance of such plates can be matched such that said criticalfrequency f obeys the condition where is the constant bending wavevelocity over the region I. That constant velocity c in turn, must beadjusted such that the desired angle of maximum radiation is given bythe relationship According to a preferred embodiment of the invention, asuitable multi-layer structure for producing this desired performance isconstructed in sandwich form, as shown in FIGURE 2. The structureconsists of a pair of opposite spaced stiff skin or surfaces 1, 1'separated by an intermediate core or spacer 5, of much smallerlongitudinal stiffness. Thus the effective Youngs modulus of thesurfaces 1, 1' will be much greater than that of the spacer or core 5.

To achieve the performance of curve I of FIGURE 3, the weight of thestrips 1, 1 must be kept small and the shear stiffness of the core orspacer must be chosen to have a certain value. With the shear stiffnessof the core or spacer 5 given by the relationship in which ,u. is theshear modulus and h represents the thickness of the core or spacer 5,the condition for producing the desired phenomenon can be expressed bythe formula where M is the total mass per unit area of the strips 1, 1'.The critical frequency (c =-c is then given by the expression s/ MB 9)'II' where B is the static bending stiffness of the composite plate1-5-1. Under such circumstances the directivity angle will remainsubstantially constant over a broad band of frequencies above thecritical frequency, as indicated at II in FIGURE 4.

As an illustration, when a paper-honeycomb spacer 5 of one inchthickness was employed between the three-mil aluminum foil skins 1, 1,secured thereto, a speed 0 of about twice the speed of sound in air anda critical frequency of about f =500 cycles per second in air wasobtained. The composite panel 1-51 required a length of about two feetto maintain substantially constant directivity down to the criticalfrequency f,,. The device served a a very adequate loud speaker foraudio frequencies applied at the driver 3.

The radiation efficiency can be increased if the width w of the strip isincreased. According to Equation 3, however, increased width means areduced upper limiting frequency. Hence it is useful to applyanisotropic core layers with a higher shear stiffness in the directionnormal to the axis of the strip, so that bending wave velocity andwavelength are increased in this direction. The honeycomb type of coreor spacer 5, for example, in view of its few cell junctions 5,inherently provides a shear modulus of rigidity in the Y direction,perpendicular to the longitudinal axis X of the panel 151; but parallelto the planes of the surfaces 1 and 1, greater than that along theX'axis.

Damping the free or right-hand end of the panel 1-5-1 in order to obtainthe necessary non-reflecting termination, before discussed, wouldinvolve dissipation of vibrational energy and reduced efficiency of thedevice. It is, therefore, advantageous to keep the weight of thecomposite panel 151 sufficiently low that the radiation damping amountsto between five and ten decibels along the length L of the panel. Littleor no additional damping is therefore necessary at the free end. Thebendingwave input impedance should be of the same order of magnitude asthe total radiation impedance of the panel. This results in bending-waveamplitudes decreasing with distance, producing a widening in the mainlobe of the directional radiation or reception pattern. Since, however,the side lobes of the pattern are reduced at the same time, this willgenerally be considered advantageous.

The angular width of the radiation or reception pattern main lobedepends, as mentioned before, upon the ratio of strip length L andbending wave length A Hence the angular Width of the main lobe willdecrease as frequency increases. This frequency response of theradiation pattern involving constant elevation angles of maximumradiation but frequency-dependent width of the maximum, is at leastpartly suppressed when the motion of the strip is controlled byvibrational-Wave or radiation damping. Since the radiation dampingincreases with increasing ratio w/x the damping increases withincreasing frequency so that the effective length of the strip isreduced. In other cases, a constant radiation pattern can be obtained bysuitable damping treatments applied to the multi-layer strip, asdiscussed in the said Letters Patent.

Forward and rearward radiation or reception is provided in FIGURE 5, onthe other hand, by isolatingly and freely suspending the composite panel1-5-1 of FIGURE 2, as at suspensions 6, within an opening 2 in a panelmounting structure 4.

Directivity patterns of almost any kind can be obtained by suitablearrangements of several composite panels 151 attached to the same or toseparate transducers 3. Thus several panels 151 are shown suspended at 6in FIGURE 6 within a plurality of substantially parallel spaced openings2 in a panel mount 4, yielding a uni-directional pattern.

Higher efficiency, attained by increased radiating surface, can beeffected by arranging several panels 151' in one plane like radii of acircle or sector, or by arranging them as shown in FIGURE 7 upon thesurface of a cone mount 4, so that radiation into all or part of theplane normal to the axis of the cone is obtained.

The panel 1-51 may also be formed in circular fashion, as shown inFIGURE 9, with the transducer 3 coupled to its center, thereby producinga cone-shaped directivity pattern.

If the panel of the present invention is sufficiently damped by itseffective mass, it is also possible to detect or radiate differentsignals under complementary angles, as, for example, by coupling atransducer 3 to each end of the panel 1-5-1, FIGURE 8. These two signalsmight comprise two components of a stereophonic sound recording, as anillustration. Stereophonic reproduction of this kind would provide thewanted impression to a listener or observer in a position very close tothe panel in the range of intersection of the two beams, as well as to alistener or observer in a more remote position, when the strip isarranged between reflecting walls, as indicated in FIG- URE 8. The twoside wall-reflected sets of waves are respectively shown in solid anddotted lines reaching opposite ears of the observer.

Other types of surfaces than the sheets or plates 1, 1 may, of course,be employed, as may other types of intermediate regions or spacers orcores 5, though the relative parameters and properties thereof mustsatisfy the criteria above set forth in order to produce the phenomenaunderlying the invention. Further modifications will also occur to thoseskilled in the art and all such are considered to fall within the spiritand scope of the invention as defined in the appended claims.

What is claimed is:

1. A broad-band directional structure for transmitting or receivingacoustic waves at an angle 5 of maximum acoustic response over a band ofacoustic frequencies above a crtical frequency f said structurecomprising an acoustic panel having a pair of stiff longitudinallyextending skin layers spaced apart by an intermediate core, andtransducer means coupled to a portion of said panel for interchangingelectric and acoustic energy in said band of acoustic frequencies, saidpanel having a length L extending from said portion and said core havinga longitudinal stiifness along said length L which is much less thanthat of said layers, said core having a shear stifiness S along saidlength and said panel having a mass per unit area M correlatedsubstantially in accordance with the following expressions c '=c /cos/S/M whereby is the velocity of bending waves along the length L oversaid band and is substantially constant,

and c is the velocity of acoustic energy in the medium surrounding saidstructure, and

where B is the static bending stiffness of said panel, the length L ofsaid panel being equal at least to c /f and said structure having adirectivity pattern with said angle (1: of maximum acoustic responsesubstantially constant over said band of acoustic frequencies.

2. The structure of claim 1, said core having a shear modulus along itswidth perpendicular to said length L which is substantially greater thanits shear modulus along said length L, whereby the velocity of bendingwaves along said width is greater than along said length.

3. The structure of claim 1, wherein said panel is elongated and saidtransducer means is coupled thereto near one end thereof.

4. The structure of claim 1, further comprising support means having anaperture and means for freely suspending said panel in said aperture.

5. The structure of claim 1, wherein said panel has means for producingradiation damping along said length L 6. The structure of claim 1,further comprising another panel like and spaced from thefirst-mentioned panel and having transducer means coupled thereto.

7. The structure of claim 6, said panels being disposed along asubstantially conic surface.

8. The structure of claim 1, wherein said panel is substantiallycircular and has said transducer means coupled to the central portionthereof.

9. The structure of claim 1, further comprising additional transducermeans coupled to a portion of said panel spaced from the first-mentionedportion.

10. The structure of claim 1, further comprising additional transducermeans coupled to a portion of said panel spaced from the first-mentionedportion, said transducer means being connected to sources ofstereophonic signals.

References Cited UNITED STATES PATENTS 1,740,651 12/1929 Huetcr l81-3 12,063,944 12/1936 Pierce 81-26 2,063,945 12/1936 Pierce 181-05 RODNEY D.BENNETT, Primary Examiner. BENJAMIN A. BORCHELT, Examiner. M. F. HUBLER,Assistant Examiner.

1. A BROAD-BAND DIRECTIONAL STRUCTURE FOR TRANSMITTING OR RECEIVINGACOUSTIC WAVES AT AN ANGLE 0 OF MAXIMUM ACOUSTIC RESPONSE OVER A BAND OFSTRUCTURE FREQUENCIES ABOVE A CRITICAL FREQUENCY FC, SAID STRUCTURECOMPRISING AN ACOUSTIC PANEL HAVING A PAIR OF STIFF LONGITUDINALLYEXTENDING SKIN LAYERS SPACED APART BY AN INTERMEDIATE CORE, ANDTRANSDUCER MEANS COUPLED TO A PORTION OF SAID PANEL FOR INTERCHANGINGELECTRIC AND ACOUSTIC ENERGY IN SAID BAND OF ACOUSTIC FREQUENCIES, SAIDPANEL HAVING A LENGTH L EXTENDING FROM SAID PORTION AND SAID CORE HAVINGA LONGITUDINAL STIFFNESS ALONG SAID LENGTH L WHICH IS MUCH LESS THANTHAT OF SAID LAYERS, SAID CORE HAVING A SHEAR STIFFNESS S ALONG SAIDLENGTH AND SAID PANEL HAVING A MASS PER UNIT AREA M CORRELATEDSUBSTANTIALLY IN ACCORDANCE WITH THE FOLLOWING EXPRESSIONS: